A simple model calculation demonstrates that phase separation of components with different net charges in spherical mixed lipid vesicles leads to huge dipole moments. Experiments on mixed lipid vesicles composed of 50% dimyristoyl-l-alpha-phosphatidylcholine (DMPC) and 50% dimyristoylphophatidic acid (DMPA) demonstrate a large permanent dipole moment of about 8.1 x 10(-24) C m (drop 2.4 x 10(6) D), which appears below a phase transition at about 30 degrees C. This phenomenon is characteristic of the mixed lipid system; it has not been observed for vesicles prepared from either DMPC or DMPA alone.